Yogananda described the centers on the cerebrospinal axis as coccyceal, sacral, lumbar, thoracic, cervical, medullary, Christ Consciousness (point between the eyebrows), and Cosmic Consciousness center (brain). But this does not mean you must be a Christian to believe in this. He saw them as whirling wheels that circulate life force (prana) throughout the body. Another great thing about all of this is, it doesn't matter what religion you practice, or what you believe.

Plato was yet another famous person who wrote about dual realities and how the physical world was based on an invisible, spiritual structure.

“The ancient Greeks considered the pineal as the seat of the soul, a concept extended by Descartes, who philosophically suggested that this unpaired cerebral structure would serve as an ideal point from which the soul could exercise its somatic functions. Descartes thus attributed to the pineal a prominent function in uniting the immortal soul with the body.” (Miles & Philbrick, 1988, pp. 409-410)

The Tree of Life, which dates back to the ancient Hebrews, is a great “snapshot” of the Chakra system, from the Yod (head) to the Heh (chest), to the Vau (belly) to the Heh (genitals and down through the feet). There is a lot of talk of spheres, orbs, space—and all of this also pertains to the Chakra “centers” in the spiritual bodies.

 

One of my favorite definitions of the word Chakras is provided by the Master of Yoga himself, B.K.S. Iyengar (Left: Perfection, in Tolasana). In his book Light on Yoga (first edition 1968), which I have considered the Bible of Yoga since I bought it in 1970 (if you don't own this book, you are really missing out on the best book on Hatha Yoga), he includes his definition of Chakras:

“The human body is a miniature universe in itself. Hatha is composed of the syllables HA and THA which mean the SUN and the MOON respectively. The solar and lunar energy is said to flow through the two main nadis, Pingala and Ida, and move down to the base of the spine. Pingala is the nadi of the Sun, Ida is the nadi of the Moon. In between them is the Susumna, the nadi of fire. Susumna (he drops the h) Nadi is the main channel for the flow of nervous energy and it is situated inside the spinal column. Pingala and Ida intersect each other and also Susumna at various places. These junctions are called Chakras or wheels and regulate the body mechanism as fly-wheels regulate an engine.”

“The Chakras are the regions situated within the spinal column where the nadis cross each other. There are 7 of them in the human body. Though they are compared to the various plexi, it should be taken for granted that the plexi alone are not the Chakras.”

“Chakras are wheels or circles. When electricity is generated, it is necessary to have transformers, conductors, fuses, switches and insulated wires to carry the power to its destination ...without these the electricity generated would be lethal.”

Kirpal Singh in The Crown of Life writes about the Plexuses and Chakras: “Wherever several nerves, arteries or veins interlace each other, that point or center is called a plexus. Similarly, there are plexuses or centers of vital forces in the Suksham (same as susumna or sushumna) or subtle nadis and these are called Chakras or padmas.”

 

In The Gospel of Sri Ramakrishna, there is a wonderful definition of Kundalini (much more on this site about this), which sums it up for me:

“The Serpent Power is the spiritual energy lying dormant in all individuals. According to the Tantra there are 6 centers in the body, designated as Muladhara, Svadhisthana, Manipura, Anahata, Visuddha, and Ajna. These are the dynamic centers where the spiritual energy becomes vitalized and finds special expression with appropriate spiritual perception and mystic vision.” (He is not including the 7th Chakra in this list. Many don't.)

Now, remembering about the susumna and the ida and pingala ... He adds, “These centers, placed in the Susumna, form the ascending steps by which the Kundalini, or spiritual energy, passes from the foot of the spine to the cerebrum.”

And the best part is that you have the power to awaken this dormant spiritual energy, or whatever you want to call it—with caution and immense respect—to play the major role in your own self-realization. He continues: “When an easy pathway is formed along the susumna through these centers, and the Kundalini encounters no resistance in its movement upward and downward, then there is the penetrating of the 6 Chakras, or mystic centers.”

Dr. Ramamurthi Mishra (Swami Brahamananda, who was a truly enlightened Indian Guru and medical doctor) in his book, Fundamentals of Yoga (1959) identified the Chakras as neurohormonal mechanisms that can be accessed through suggestion. In his view, the Chakras are ganglionic centers that are related to endocrine gland function, as shown in his following table:

CHAKRA	CONTROLS	ASSOCIATED GLAND	NEURAL PLEXUS
Muladhara	sex organs	gonads/ovaries	perineal
Svadhisthana	legs and feet	adrenal glands	sacral
Manipura	abdominal organs	pancreas	lumbar
Anahata	breathing and heartbeat	thymus	thoracic
Vissudha	arms, hands and speech	thyroid	cervical
Ajna (a)	entire body	pituitary	thalamic
Sahasrara (b)	entire body	pineal	cerebral cortex

Other sources assign a different correspondence of glands to chakras:

• the adrenals are relegated to the root chakra
• the gonads/ovaries to the navel chakra
• the pineal as the gland of the forehead chakra
• the brain chakra, the pituitary.

Dr. Mishra locates them in the spinal cord and central nervous system, which makes a lot of sense to me. There are many theories about Chakras. It doesn't really matter who is right. We just need to acknowledge that they do exist and that we can work with them—even if they are not visible to us (some people do see them clearly).